Hurricane Season Begins With New Plans And Old Levees

Oct 1, 2009

It seems like just yesterday when Hurricane Katrina, with her huge footprint and devastating storm surge, flooded New Orleans and devastated the Gulf Coast. Get ready for round two: Hurricane season officially starts today.

Over the past seven months, over 150 of the nation's leading scientists and engineers from government, academia and private industry have been painstakingly studying what, exactly, caused the levees to fail last August. Today, the Army Corps of Engineers, along with several other national, state and university research teams, will release their findings and recommendations on how to better defend New Orleans from the inevitable: Another storm, perhaps greater in size and power than Katrina, hitting with little notice. It hasn't been an easy task.

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The Interagency Performance Evaluation Task Force (IPET)—the Corps team in charge of coordinating the analysis of Katrina's failures—used the sophisticated technology, from supercomputers that model the storm surge experienced during hurricane conditions, to hanger-sized models of sections of levee in an effort to better understand how New Orleans' floodwalls and levees performed during Katrina—and how they failed.

To simulate the hurricane forces and water levels that led to breaches in the 17th Street and London Avenue Canals on August 29, 2005, the U.S. Army's Engineer Research and Development Center (ERDC) constructed two 1:50 scale models to duplicate the failed sections of floodwall, using sand and peat sampled from the sites in order to create as realistic model as possible.

ERDC also built a massive, 14,500-squarefoot model of the 17th Street Canal—the floodwalls on the drainage channel--which cuts into the heart of the city—experienced one of the worst breaches during the storm. The model replicates ½ mile of the canal and more than a mile of Pontchartrain lakefront and it cost approximately $400 thousand to build. "It's one-third the size of a football field," says IPET's Wayne Stroupe. "It's used for wave modeling. It allows us to recreate the waves and determine their effects on the canal during the storm."

The results of the tests were no big surprise to the engineers. Researchers placed the smaller models in a centrifuge, spinning them to establish a force 50 times that of the Earth gravity, and poured in water to recreate a Katrina level environment. Video recordings of the experiment show the 17th Street Canal floodwall's clay foundation shearing, the I-Wall healing over under the force of the surge and the waves, the walls soil backing foundation displacing and the clay eroding further. It is, essentially, just what the walls supposed to not do. Needless to say, the test was consistent with reality. (Click here to download video of the testing, sped to 4x speed, in Windows Movie Player format.)

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The London Avenue canal model met a similar fate in the centrifuge. According to an ERDC presentation, "as the water level rose to flood levels, a crack formed down the front of the wall. Failure of the wall is associated with an increase in the hydrostatic pressure (uplift) under the downstream levee."

While IPET has been busy in the lab, and continues to use the data to evaluate the performance of the levees, the Army Corps of Engineers' Task Force Guardian, has been working furiously to meet today's deadline to rebuild and repair the current floodwall and levee system to a "pre-Katrina" level.

"There are miles and miles of levees." Stroupe said. "And most have been repaired."

Of the nearly 350 miles of levees ringing the bathtub city, Stroupe estimates that Task Force Guardian has completed approximately 95 percent of repairs at this time. A major project—and one, had it existed when Katrina crossed over the crescent city, might have spared the 17th Street and London Avenue Canals—are outfall gates sealing the drainage canals from a storm surge on Lake Pontchartrain.

Before Katrina, many argued that outfall gates would prevent the city from pumping floodwater out, into the drainage canals and Lake Pontchartrain. To imagine the topography of New Orleans is to envision a salad bowl sunk into a shallowly filled sink. The water level in the sink lies just below the edge of the bowl. Turn on the faucet and raise that water level—as Katrina did, by many orders of magnitude—and the surrounding water becomes perilously close to rushing in and swamping your salad. Many areas of the city are below sea level and during heavy rainfall the outfall gates must remain open to aid in pumping and avoid flooding. But when a major storm is predicted the gates will close to protect the city from storm surge. Stroupe expects the project to be completed between late-June and early-July.

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Getting New Orleans's protection system back where it was before Katrina has been a hurculean task by any measure, but it pales to the challenges ahead. By September 2007, all levees must meet their authorized design specifications. By studying the data gathered by IPET and Task Force Guardian, researchers discovered that some levees no longer met the requirements of their original designs.

According to Stroupe, over the years, the levees have deteriorated, and much of the sediment they were built on has settled for any number of reasons, so construction will continue until all levees are the exact height they were intended to be and precautions have been taken against future changes. But the real work will come even after that: By December 2010, all levees must carry a 100-year flood mark certification, a far more robust standard that requires the entire system to withstand a storm the likes of which we see no more than once every hundred years.

Which begs the question: Was Katrina the storm of our century, or simply the overture?—Ted Latiak

Deconstructing Disaster: The Army Corps of Engineers pulls breached sections of the 17th Street Canal floodwall foundation from the New Orleans mud in December. After seven months of study, the Corps is prepared to release its findings today. Photo by B. Chertoff

Reconstructing Disaster: Engineers from the Army and from Rensalair Polytechnic Institute build 1/50th scale models of the 17th Street and London Avenue canals. Army Corps of Engineers

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Oversize Expectations: Even in December, 2005, Members of Task Force Guardian rush to repair levee breaches in time for today's deadline: The official start of the hurricane season. Photo by B. Chertoff

Foundation of Failure: An Army Corps of Engineers document illustrating likely failure methods for the 17th Street Canal floodwall. Army Corps of Engineers

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